Class GranuleDescriptor

A granuleDescriptor is a single piece of the mosaic, with its own overviews and everything.

This class is responsible for caching the various size of the different levels of each single
granuleDescriptor since computing them each time is expensive (opening a file, looking for a
reader, parsing metadata,etc...).

Right now we are making the assumption that a single granuleDescriptor is made a by a single
file with embedded overviews, either explicit or intrinsic through wavelets like MrSID, ECW or
JPEG2000.